Most players of the French Defence know, that a white bishop on d3 is especially dangerous for their king, because White can often sacrifice the bishop on h7 and start a devastating mating attack by doing so.
In the position after White's 9.Bd3 this threat is actually acute, because if Black blunders now (e.g. 9...c4??) White can sacrifice the bishop on h7 and gets a mating attack.
The cheap defense against this type of bishop sacrifice (9. ... h6?) is met by Bxh6, and (9. ... g6?) allows White to open the h-File with ease (h4 -> h5). That's why there is really only one good defence Black can play: 9...f5